Food products are dehydrated to: (1) increase their shelf-life, and (2) to avoid the deterioration of aroma and flavour as well as nutrient degradation. This is because water is known to degrade food products by oxidization; infection by microorganisms; destruction of proteins, nutrients, and food enzymes; and destruction of the food colors, forms, and flavors. Vacuum freeze drying apparatuses are selected to remove water (dehydrate) from food products. Under the dehydration process of the vacuum freeze drying apparatuses (lyophilazation), water in the food products are removed under the low temperature and pressure condition.
There are four basic components to the conventional vacuum freeze drying apparatus. They are: a freeze dried chamber, a condenser unit, a refrigerator unit, and a vacuum pump unit. Each plays a vital role in the functionality of the conventional vacuum freeze drying apparatus. In the freeze dried chamber, products are either laid flat in trays or contained in vials. The surfaces of the products make direct contacts with the products so that heat transfers can occur. The refrigerator unit provides cold temperatures to the condenser which, in turn, provides cold temperatures to the surfaces of the freeze dried chamber. Finally, the vacuum pump unit is designed to bring the products to the triple-point (sublimation) temperature (TSUB) where ice sublimates into vapours.
In such conventional vacuum freeze-drying apparatus, food products are undergone four different operating stages. In the primary drying stage, the product temperature is firstly decreased in such a way that all the free water freezes. In the secondary drying stage, the product is exposed to low pressure and the ice sublimates. In the post drying phase, or at the end of the sublimation stage, the amount of residual water can be further decreased by removing the bound water. This stage is generally carried out increasing product temperature and decreasing the operating pressure. This way, all the characteristics of the product, e.g. shape, appearance, colour, taste and texture, are retained in the final product.
However, the conventional freeze drying apparatuses is replete with many drawbacks, especially in the industrial applications. One main drawback is the nonuniformity of the cold temperature zones in the freeze dried chamber that cause nonunformity in the final products. Another drawbacks are the cost and efficiency because the energy required by the lyophilazation process is higher with respect to that of other drying devices. Yet another drawback is the lack of automatic controls that lead to difficulties in freeze drying of different products having different properties.
More particularly, conventional industrial-size vacuum freeze drying apparatuses generate non-uniform freezing patterns of temperatures and pressures. That is, within the large refrigerated area, freezing patterns of different temperatures and pressures are formed due to the uneven heat transfer caused either by (1) the forced conduction of the fans or (2) by none at all. The freezing patterns are proportional to the area and the distance of the chamber. The closer to the fans the stronger the heat transfer by the forced conduction. To solve this distribution problem, multiple fans are used to reduce the segmented freezing patterns. However, multiple fans consume more electrical energy, hobbling efficiencies and increasing costs. In addition, pockets of freezing patterns are formed in the overlapping areas between the fans. This problem becomes less severe as the vacuum freezing process proceeds because it takes time for air to diffuse in order to reach equilibrium. However, at the time the temperature and condition inside the condenser reach equilibrium, the freeze dried food batch have changed differently in different sections of the chamber. Therefore, the food products being freeze dried are not uniform. Most of the time, if it takes too long to reach the triple point temperature, the aroma and flavor of foods will be degraded. The taste, the texture, the flavor, and the essence are no longer the same as the original food products.
Furthermore, when multiple fans are used, ice crystals are formed on the fans at the end of the freezing process. Thus, conventional vacuum freeze drying apparatus demands additional heat energy to defrost these ice crystals, thus consuming more energy and degrading the efficiency of the entire process. The efficiency depends on the input energy, the output energy, and the duration of the entire process. If the vacuum freeze drying process is not performed properly, the following problems may occur: (1) when the cooling rate is not sufficiently fast, the formation of large ice crystals can cause the freeze dried products to be brittle and destroy the microscopic structures of product; (2) when the cooling temperature is not below the eutectic temperature (Teu), water and unwanted solutes will not be completely removed, rendering the entire process ineffective; (3) when the pressures and temperatures are not carefully controlled, the product may collapse, destroying the essence of the product. In addition, the conventional vacuum freeze driers are largely controlled by human interfaces which cause the above problems; and (4) each product requires different eutectic temperatures (Teu), optimal temperatures (Topt), pressures, and settings for being properly freeze dried. For example, the freeze drying of water melons is different from that of walnuts because water melon contains more liquid than walnuts. Using generic settings for different products would likely render the freeze drying process of different products ineffective and uneconomical.
Therefore, what is needed is a vacuum freeze drier that can provide a deep and uniform freezing pattern inside the chamber in a relatively short time to capture the essence of the products and to achieve uniformity.
What is needed is a vacuum freeze drier and method that are fully automatic, i.e., controlled and observed by a controller unit or a computer that can create optimal freeze drying conditions for each specific product.
What is needed is a vacuum freeze drier that can provide a high rate of cooling so that the microscopic structures of the product are preserved.
Furthermore, what is needed is a vacuum freeze drying apparatus that can reuse the water vapors to provide heat to the product in the dryer unit so that energy is conserved and the entire vacuum freeze drying process is efficient.
Yet, what is needed is a vacuum freeze drying apparatus that can provide specific settings including eutectic temperatures (Teu), optimal temperatures (Topt), pressures, and cooling rates for specific fruits having different aqueous sucrose levels so that structural collapse can be avoided.
Finally, what is needed is a method of operating such vacuum freeze drying apparatus so that the above objectives can be achieved.
The vacuum freeze drying apparatus disclosed in the present invention solve the above described problems and objectives.